UPDATE 2:40 p.m.: The House has rejected the Senate amendment, as expected.

Today is the House of Representatives’ last day in session before departing for an August recess full of photo ops and electioneering in their districts. The Senate will stick around DC for one more day before going home. Before that happens, the two houses have to come together on a plan to keep the Highway Trust Fund going. If not, U.S. DOT will have to take drastic measures[1].

[2]

Republican Sen. Bob Corker disagrees with the House GOP on when the bill should expire and how to pay for a new one.

Both the House[3] and the Senate[4] have voted on not entirely dissimilar plans to keep the fund going. But the differences between them have set up a high-stakes showdown that has to be resolved by tomorrow.

Here are the key points:

The timing: The House is expected to vote on the Senate bill today at about 3:00 p.m. and is expected to refuse to budge. Then they’ll leave town, meaning the Senate can either cave or be blamed as the Highway Trust Fund goes dry before August recess ends and transportation works grind to a halt. Meanwhile, Sec. Anthony Foxx has warned state DOTs[1] that federal payments will slow down August 1 — that’s tomorrow — if Congress doesn’t take action to keep the Fund from going insolvent.

The numbers: The House is gloating[5] that the Senate’s bill contains a $2 billion technical error — which is true; it comes up with just $6.2 billion of the $8.1 billion needed — but Senate Democrats say it can be easily fixed.

The urgency: Since summer is the high season for construction, the real pressure on the Highway Trust Fund is between now and the end of the year, when states will need to get reimbursed for the work that’s going on now. That’s why there’s not a huge monetary difference between the House proposal that lasts till May and the Senate proposal that ends in December. There’s just not a lot of cash going out the door at U.S. DOT between January and May.

The conflict: The House and Senate disagree on what budget gimmicks[6] to use to “pay for” the transfer into the trust fund, but more fundamentally they disagree about how long the patch should be. As we’ve reported before[7], Boxer prefers a December deadline, saying it’s unfair for this Congress to fail to fix a problem that occurred on its watch and instead kick it to the next Congress. What she means is that she wants her six-year bill[8] to pass and that won’t happen after the end of this year if the GOP wins a majority in the Senate and she loses the chairmanship of the EPW Committee. That’s precisely why the House is gunning for a May deadline.

The breakdown: The Senate Republicans aren’t as enthusiastic as the House about having to take this up when they’re in charge. Thirteen Rs joined the Ds in pushing for a December sunset, including Sen. Bob Corker (R-TN), who wants to raise the gas tax and be done already. “Wouldn’t it be great to finish 2014 actually solving one issue; taking one issue off the plate next year?” he said yesterday[9] at a WSJ press breakfast. Only one Democrat, Jeanne Shaheen of New Hampshire, voted no on Boxer’s date-change amendment. Notably, David Vitter, the ranking member on the EPW Committee, who has shown great bipartisan unity with Boxer, broke with her on this and voted to essentially flush their six-year-bill down the toilet. His predecessor, James Inhofe, voted in favor of Boxer’s December 19 deadline.

The fallout: If the GOP does win the Senate in 2014, the conventional wisdom says they’ll lose it again in 2016. Will the Republicans really want to take on a tax increase of any kind during the only two years when they’ll get the lion’s share of the blame? Of course not. The prognosis is that if there’s no long-term bill this term, it’ll be another three years. Three more years of patchwork funding gimmicks is nothing to look forward to.